After World War II, the city again became part of Poland, as it fell within the new borders determined by the Potsdam Conference.

Slavic names in Pomeranian — Stolpsk,[4] Stôłpsk, Słëpsk, Słëpskò, Stôłp[5] — and Polish — Słupsk — may be etymologically related to the words słup 'pole' and stołp 'keep'.

There are two hypotheses about the origin of those names: that it refers to a specific way of constructing buildings on boggy ground with additional pile support, which is still in use, or that it is connected with a tower or other defensive structure on the banks of the Słupia River.

This factor led to the construction of a grod, a West Slavic or Lechitic fortified settlement, on an islet in the middle of the river.

Archaeological research has shown that the grod was situated on an artificial hill and had a natural moat formed by the branches of the Słupia, and was protected by a palisade.

Records confirm that the area of Słupsk was part of the Polish realm during the reign of Mieszko I and in the 11th century.

[7] According to several sources, the first historic reference to Słupsk comes from the year 1015 when the king of Poland Boleslaus I the Brave took over the town, incorporating it into the Polish state.

[9] Another document dated to 1180, which mentions a "castellania Slupensis" and would thus be the oldest surviving record, has been identified as a late 13th-century or 14th-century duplicate.

After Mestwin II's death the city was reintegrated with Poland and remained Polish until 1307, when the Margraviate of Brandenburg took over, while leaving local rule in the hands of the Swenzones dynasty, whose members were castellans in Słupsk.

[16] In 1337, the governors of Słupsk (Stolp) had purchased the village of Stolpmünde (modern Ustka)[7] and then constructed a port there, enabling a maritime economy to develop.

[7] The Polish language in general, however, was experiencing very unfavourable conditions due to depopulation of the area in numerous wars and implied Germanization.

[20] After the Thirty Years' War, Stolp lost much of its former importance—despite the fact that Szczecin was then ruled by Sweden, the province's capital was situated not in the second-largest city of the region, but in the one closest to the former ducal residence—Stargard.

However, its geographical location led to rapid development, and in the 19th century, it was the second city of the province in terms of both population and industrialization.

In 1769, Frederick II of Prussia established a military school in the city, according to Stanisław Salmonowicz its purpose was the Germanization of local Polish nobility.

Demographic growth remained high, although development slowed, because the city became peripheral, the Kreis (district) being situated on post-war Germany's border with the Polish Corridor.

During the war, Germans brought forced labourers from occupied and conquered countries and committed numerous atrocities.

People in the labour camp were maltreated physically and psychologically and forced to undertake exhausting work while being subject to starvation.

[26] Between July 1944 and February 1945, 800 prisoners were murdered by Germans in a branch of the Stutthof camp located in a railway yard in the city; today a monument honours the memory of those victims.

[citation needed] After the war, the city became again part of Poland and most of the German population either fled or was expelled in accordance with the Potsdam Agreement.

It was initially part of Okręg III, comprising the whole territory of the former Province of Pomerania east of the Oder River.

In 1945, the first post-war craft workshops and public schools were opened, trams and a regional railway started to operate, and the amateur Polish Theater was established.

[28] The puppet theatre Tęcza used to collaborate with the similar institution called Arcadia in Oradea, Romania, but the partnership ceased after 1989.

The city has a fairly irregular shape, with its central point at Plac Zwycięstwa ("Victory Square") at 54°27′51″N 17°01′42″E / 54.46417°N 17.02833°E / 54.46417; 17.02833.

[34] The city lies in a zone where the continental climate influences are very weak compared with other regions of Poland.

Słupsk is a railway junction, with four lines running north, west, east and south from the city.

[41] The city has rail connections with most major cities in Poland: Białystok, Gdańsk, Gdynia, Katowice, Kraków, Lublin, Łódź, Olsztyn, Poznań, Szczecin, Warsaw and Wrocław, and also serves as a junction for local trains from Kołobrzeg, Koszalin, Lębork, Miastko, Szczecinek and Ustka.

The bypass is a part of Expressway S6 which, when completed some time after 2015, will give Słupsk a fast road connection to Szczecin and Gdańsk.

Słupsk is the regular venue for a number of festivals, most notably: For a long time here lived Anna Łajming (1904–2003), Kashubian and Polish author.

Before the end of World War II, the vast majority of the town's population was composed of Protestants.

[48] On February 12, 2016, the US Army has awarded AMEC Foster Wheeler a $182.7 million contract with an option to support the Aegis Ashore missile defense system in Poland.